Modulation synthesizer circuits are commonly used in radio frequency (transceivers) transmitters. In conventional modulation synthesizer circuits, harmonic frequencies generated at the in-band intermediate frequency (IF) will alias back into the transmit spectrum at the voltage controlled oscillator due to the inherent sampling/limiting in the phase-lock loop up converter. Thus, conventional modulation circuits require the use of a bandpass filter to filter out the undesirable harmonic frequencies at the intermediate frequency. However, bandpass filters typically consist of bulky and expensive components such as capacitors and inductors. Consequently, the use of a bandpass filter to filter harmonic frequencies increases the cost and complexity of the modulation synthesizer circuit. Moreover, because of the associated size and space demands of the components required in bandpass filters, it is difficult to place the entire modulation synthesizer circuit on a single integrated circuit.
A block diagram of a conventional modulation synthesizer circuit 10 using a bandpass filter 70 is illustrated in FIG. 1. Modulation synthesizer circuit 10 includes a transmit voltage controlled oscillator (VCO) 25 which delivers a modulated spectrum to output terminal OUT as well as to a down conversion mixer 40. The local oscillator (LO) input to the mixer 40 is driven by a frequency synthesizer 30, which has a phase-locked loop circuit 32 and a voltage controlled oscillator 34. Frequency synthesizer 30 produces a carrier wave signal as LO input to down conversion mixer 40. The output of down conversion mixer 40 will be a copy of the modulated spectrum centered at an IF frequency that is the difference between the center frequency of the modulated signal from transmit VCO 25 and the frequency generated by frequency synthesizer 30.
A phase shifter 50 is conventionally located after mixer 40, within the modulation synthesizer loop 80. Phase shifter 50 receives the IF modulated signal from mixer 40 and produces two fixed IF signals having the same frequency but that are 90 degrees out of phase with each other, otherwise known as fixed IF quadrature signals.
The two fixed IF quadrature signals from phase shifter 50 are received by a quadrature modulator circuit 60, which produces a signal with a fixed IF unmodulated frequency. Quadrature modulator circuit 60 includes two mixers 62 and 64, as well as a summing circuit 66. Quadrature modulator circuit 60 also receives an in-phase base band signal at terminal I as well as a quadrature base band signal at terminal Q, which is 90 degrees out of phase with the in-phase base band signal. The in-phase base band signal and quadrature base band signal at I and Q input terminals, respectively, are complex conjugates of the modulated input spectrum produced by phase shifter 50. Thus, quadrature modulator circuit 60 produces a fixed IF unmodulated signal at its output terminal. The fixed unmodulated signal is then filtered by a anti-aliasing bandpass filter 70.
Modulation synthesizer circuit 10 also includes a wide band phase-lock loop (PLL) circuit 20 with an input terminal REF, which receives a reference signal typically generated by an IF synthesizer or from a crystal. Wide band PLL circuit 20 has a second input terminal FEEDBACK, which receives the fixed IF unmodulated signal from the output of quadrature modulator 60 filtered by a bandpass filter 70. Wide band PLL circuit 20 compares the two input signals by dividing the reference signal by R (an integer) and the fixed IF unmodulated carrier wave signal by N, and produces an output signal having a voltage level that ensures that the phase difference between the two signals is minimized. The control input terminal of transmit VCO 25 receives the output signal from wide band PLL circuit 20 after it is filtered by a loop filter 22. The transmit VCO 25 then up converts the signal received from loop filter 22, which results in the modulated spectrum at the output terminal of transmit VCO 25.
If a change in frequency is imposed on the signals at the I and/or Q input terminals, the frequency of the fixed unmodulated signal produced by quadrature modulator 60 will change. Consequently, there will be a phase difference between the signals at the input terminals of the wide band PLL circuit 20. The wide band PLL circuit 20 will then generate an output signal which will change the frequency of the modulated spectrum generated by transmit VCO 25. The frequency of the modulated spectrum generated by transmit VCO 25 will be such that the IF modulated signal produced by mixer 40 will be the complex conjugate of the new frequency imposed at the I and/or Q input terminals.
Similarly, a change in the frequency produced by frequency synthesizer 30 will force the frequency at transmit VCO 25 to change correspondingly, because the wide band PLL 20 will force the FEEDBACK input to remain at a fixed frequency. Therefore, the center frequency of the modulated transmit signal at OUT can be changed by changing the LO frequency produced by frequency synthesizer 30.
Unfortunately, phase shifter 50 does not only generate output signals at the IF frequency, but also generates harmonics of the fixed IF frequency, as shown in FIG. 2. FIG. 2 is a graph showing the spectrum 82 produced by phase shifter 50 at the IF frequency as well as the spectra 84, 86 at third and fifth harmonics of the IF frequency, respectively. The instantaneous IF frequency is offset from the output of the quadrature modulator 60 by the complex conjugate frequency generated by the signals received at the I and Q input terminals, so the frequency component of the third harmonic will be three times the complex conjugate frequency away relative to three times the frequency of the signal produced by quadrature modulator 60. Similarly, the fifth harmonic will be five times further away relative to five times the frequency of the signal produced by quadrature modulator 60. Therefore the spectra at the harmonics will become wider as shown in FIG. 2.
Although quadrature modulator circuit 60 produces a fixed IF unmodulated signal, it also modulates the harmonic frequencies generated by phase shifter 50. However, because the spectrum at the harmonic frequencies is now wider, as shown in FIG. 2, the spectrum at the harmonic frequencies does not correspond to the modulation imposed by the signals received at the I and Q input terminals. Consequently, quadrature modulator circuit 60 can no longer remove the modulation, as shown in FIG. 3. FIG. 3 is a graph showing the spectrum 88 produced by quadrature modulator circuit 60 at its output as well as the spectra 90, 92 produced at the third and fifth harmonic frequencies, respectively. As shown in FIG. 3, the quadrature modulator 60 produces a fixed unmodulated signal at the IF frequency, but modulated signals at the third and fifth harmonics of the IF frequency.
The signals at the IF frequency as well as its harmonic frequencies are received by wide band PLL circuit 20, which acts similar to a fast sampling device with a sampling rate equal to the phase detector frequency. Thus, the signals at the harmonics of the IF frequency produced by quadrature modulator circuit 60 will alias into the transmit spectrum, causing distortion of the desired spectrum, as shown in FIG. 4. FIG. 4 is a graph showing the spectrum generated by wide band PLL circuit 20 in which the spectra 90, 92 at the third and fifth harmonics, shown in FIG. 3, are aliased into the transmit spectrum at the transmit frequency f.sub.TX. It is understood that the spectra illustrated in FIGS. 2, 3, and 4 relate to one particular given modulation standard, and that other spectra will exist for other modulation standards.
Consequently, an anti-aliasing bandpass filter 70 is required to remove the spectra at the harmonics of the IF. However, an anti-aliasing bandpass filter with sufficient attenuation of the harmonic components has approximately four inductors and four capacitors, which are bulky and expensive in both cost and real estate when placed on a silicon chip. Thus, when modulation synthesizer circuit 10 is placed on a single silicon integrated circuit, bandpass filter 70 is generally located externally.